This invention relates to voltage arresters in general and more particularly to an improved arrangement including a plurality of series arrays of spark gaps which have associated therewith control spark gaps coupling each of the junctions between two series arrays with the upper or lower potential.
The use of metal enclosed switching installations with gas insulation capable of withstanding extremely high system voltages has within a short period of time become extremely significant in the power industry. With the steady increase in energy requirements and the associated high load densities associated therewith, the use of steadily increasing higher operating voltages for operational and economic reasons has developed. However, at the same time, the physical space available for switching installations to be constructed has become limited. By filling switching installations with an insulating gas, the area and space requirements for these installations can be reduced to approximately 10% of that required using conventional construction. All components in such installations which carry high voltage, such as conductor contacts and breaker quenching chambers are accommodated in a pole by pole manner within a metallic casing. Switching installations of this manner are often also equipped with surge voltage arresters. For general background on the use and types of surge voltages arresters known in the prior art, see Standard Handbook for Electrical Engineers edited by A. E. Knowlton, 9th ed., McGraw Hill, 1957, Section 12-138 et. seq. page 1033 et. seq.
Typical prior art surge voltage arresters, however, cannot be arranged within an enclosed installation itself.
Typically, sulfurhexafluoride is used for gas insulation. It has an electrical breakdown voltage which is approximately two to three times that of air at atmospheric pressure depending on the shape of the electrodes and the type of voltage stress. Generally, it is desireable to further increase the breakdown voltage by providing such a gas at an increased pressure of approximately 3 to 4 atm. abs. referenced to 20.degree. C. If surge voltage arresters are installed within an enclosed switching installation of this nature, their breakdown voltage will be increased accordingly by the insulation gas. However, such breakdown voltage in surge voltage arresters should be as low as possible and, in some cases, cannot exceed the peak value of the operating voltage by a significant amount, i.e., in some cases the difference between the peak operating voltage and the voltage at which the surge voltage arrester must break down is very small. Reduction of the electrode spacing to take into account the increased breakdown voltage brought about by the insulating gas is possible only to a limiting distance of approximately 1 mm. This is a direct result of manufacturing tolerances which can be attained. Such a spacing is not sufficient to reduce the breakdown voltage to the required level.
Surge voltage arresters used in high voltage switching installations are usually valve-type arresters having spark gaps which fire when a surge voltage occurs to open up a low resistance current path through the arrester. Following such, the electric arc is then extinguished and the arrester is again rendered currentless. After the main portion of the surge voltage pulse has been conducted through the arrester, but with the electric arc still burning, a power frequency follow current will continue to flow. The spark gaps in the arrester must be arranged so that they are capable of interrupting this follow current.
Various arrangements for accomplishing this have been developed. For example, the spark gaps may be designed as horn-shaped electrodes each housed in an arcing chamber with a large number of these formed into a stack and the individual spark gaps connected in a series array. With such a stack, a blasting coil can be associated having a magnetic field which will tend to stretch the electric arc which exists between each of the two horn-shaped electrodes to thereby extinguish it. In another known arrangement in which blasting coils are not provided, the electric arc is driven under the effect of its own magnetic force between arc quenching plates. In such arrangements, the spark gaps are extinguished as the voltage passes through zero.
The breakdown voltage of high voltage arresters is generally a function of the steepness of the rising edge of the shock voltage wave which hits the arrester. For pulses having a rise time substantially shorter than, for example, one microsecond, the breakdown voltage of such arresters can experience an increase of more than 20%, i.e., the arrester will not respond until its nominal breakdown voltage is exceeded by more than 20%. This increase in the breakdown voltage for voltage pulses having a sharp rising edge is caused by the discharge delay and charging time of the breakdown. For plate type spark gaps having an almost homogeneous field, the increase of the breakdown voltage is less pronounced than is the case for the generally used modern magnetically blasted arresters such as those disclosed in German Offenlegunsschrift 1,286,190. However plate-type spark gaps are not suitable for pulse voltages unless additional special measures are taken. This is because the concentration of the electrical field at the edges results in an undefined discharge delay and brings about a correspondingly great spread of the breakdown voltage.
Also known in the art are arrangements of series arrays of two spark gaps. It has been found, however, that in such arrangements the breakdown voltage will decrease with multiple operations. The drop in breakdown voltage can be limited through the use of a special control spark gap associated with one of the spark gaps or with a series array of such spark gaps. In such an arrangement, a control spark gap with a limiting resistor in series is connected in parallel to one of the series arrays. The control spark gap contains two electrodes which are shaped in the manner of a corrugated board and which are positioned opposite each other at irregular distances along their lengthwise dimension. The minimum spacing between the two electrodes becomes the breakdown voltage of the spark gap. The breakdown voltage can be adjusted within a certain range by providing means for adjusting the electrode spaces such as a set screw. An arrangement such as this is described in U.S.S.R. Pat. 126,174. With a design of this nature the decrease in the breakdown voltage due to multiple operations will be limited. However, the spread of the breakdown voltage noted above remains comparatively high since breakdown takes place with preference at the edges of the electrodes because of the substantial field concentration which occurs at those points.
In view of these difficulties, it is the object of the present invention to provide an high voltage arrester having a low and adjustable constant breakdown voltage, which high voltage arrester can be arranged in a fully enclosed, insulating gas filled switching installation of the type described above.